Relative age of the saline lakes and relation to East Africa lakes:
Groundwater salinity in the rift basins showed that they coincide with the deltas and fans which occupy the flowing end of the hydrological and hydrogeological closed basins. The deepest part of the basin at the downstream end was always a lake, sebkha or playa. The saline deposits were formed through processes of efflorescence, leaching and evaporation, leaving behind carbonate deposits as kanker nodules in the sheet flow area, and as thick deposits in the marginal fault zones.
Is it possible, in a preliminary way, to correlate the saline lakes in Sudan with similar lakes in Eastern Africa, Ethiopia and Afar.
All C14 ages determined from groundwater in the central parts of these basins, gave an age of above 40,000 years BP (Salama 1985b).
Malmberg and Abdel Shafie (1975) using O18 results of groundwater from Bara basin showed that the groundwater from the lower horizon is from rains of much wetter and cooler periods than the present, with the temperature 4 to 8 degrees C cooler.
Assuming equal rates of erosion and sedimentation throughout the sedimentary column during the Quaternary, the top 100-300 meters of sediments, which contain the saline zones, would accumulate in 10,000 to 30,000 years using the rate of sedimentation of Yuretich and Cerling (1983), and 100,000 to 300,000 years on an average rate of 1 mm year-1 .
The approximate age of the lakes was calculated by the amount of salt deposited as related to the annual flow, assuming the salt content of the inflow is 120 mg l-1 ( which is comparatively high even in today's rates), and the palaeo-flow would be ranging between 2 km3 and 20 km3 annually, then the lake age would range from 40,000 to 110,000 years.
From the rough estimates of ages made by sedimentation rates of above and the age determined by the salinity data, the approximate age of the Sudanese lakes may be comparable with the ages of the lakes Abhe I and II which is between 30 000 to 70 000 years. The Sudanese lakes can also be correlated with the wet pluvial periods of Said (1981). From this limited evidence of age, it can be postulated that the formation of those saline water bodies, would most probably be in the period between 90-120 thousand years ago, ie within the late Quaternary.
The River Nile
Egypt is the gift of the Nile. This is the first reading statement to be taught to all children in schools along the River Nile in Egypt and Sudan. The early Egyptians used to celebrate the floods by sacrificing one of their daughters in the Nile . All this respect , or fear , stems from the fact that the people wondered whence all this water came. The origin of the River Nile was a big mystery . The early Egyptians kept records of the Nile floods from 3500 B.C. The events were engraved on stone known as the broken stone Nilometer, that is now housed in Palermo Museum . Amenemhat III cut inscriptions in the rocks of Semna Catract showing the water levels of the Nile at that time (Wilcocks,1904; Ball,1939). Biswas (197 ) recorded the old history of the scholars who studied the behaviour of the Nile since Eratosthenes of Alexandria (about 200 B.C.). Said (1981) gave a list of literature on the geological history of the Nile from Hull(1896) to Said (1975). The recent scholars who studied the evolution of the River Nile include ; Adamson (1980), Williams and Williams (1978), McDougall et al.(1975), Butzer(1980), Butzer and Hansen (1968) De Heinzelin ( 1968),De Heinzelin and Paepe (1965), Wendorf and Schild (1976), Said (1981) and Issawi (1984). De Heinzelin (1968) worked out the fluvial deposits of the river at Nubia . Said and Issawi (1964), both recognized that the fluvial sediments of the river fall into two major groups.
a) Earlier sand-gravel complex, which must have been deposited in a fluvial environment under a different regimen with a different source. b) A later group of sediments that are similar to the sediments which accumulate in the present day Nile. Butzer and Hansen (1968) studied the geomorphology and sediments of the Nile valley in Nubia and Kom Ombo area. The Pliocene Gulf deposits of Sandford and Arkell (1939) described from the west bank of the Kom Ombo area are taken by Butzer and Hansen to indicate a fluvial to lacustrine environment during the Pliocene. They also identified from the east bank a shale and evaporite facies which they considerered to be of lagoonal origin (including gypsum and salt pockets). The earlier pre-late Miocene river which drained the elevated lands of Egypt , did not seem to have been associated with the valley in its present form. Relicsof these earlier rivers are preserved in fluviatile sand and gravel spreads and in deltaic sediments recorded in several places in northern Egypt. The earliest of the fluviatile Tertiary deposits date back to late Eocene and early Oligocene time in the Faium region to the north of Birkat Qarun at an elevation of 350m above sea level. The fluviatile facies deposits covering the Eocene-Oligocene deposits are of lower Miocene age and are widespread in the western desert. Along the road from the western oasis in Egypt to Salima oasis in northern Sudan , crossing the Sahara desert, the author in 1973, noticed that these sediments represent sheet flood deposits which possibly originated from Wadi Howar water shed , draining the area north of Darfur dome. The Quaternary in Egypt was an epoch of great aridity punctuated by several pluvial periods , none of these wet periods was sustained or vigorous enough to feed a Nile like the late Pliocene Paleonile or the late Miocene Eonile. On two occasions the Quaternary pluvials produced master streams, one of which was short in duration (the Proto Nile) and the other ephemeral in nature (the Abassia). The Quaternary Niles in Egypt owed their existence to the connection with the Ethiopian high lands, which took place during the middle Pleistocene and has continued off and on since then (Said 1981).
_____________________________________________________
Table 1 - WET PERIODS RECORDED BY SAID IN SOUTHERN EGYPT
_____________________________________________________
9000 B.P. (Final Paleolithic-Neolithic)-Dishna-Ineiba --Holocene pluvial.
80000 B.P (Mousterian-Aterian)- Korosko-Makhdama-pluvial-late pleistocene.
200000 B.P. (Acheulian) -Abassia pluvial-middle-Pleistocene
1.000.000 B.P. (Idfu) Complete master stream,theProto Nile- late early Pleistocene.
1.500.000 B.P. (Armant) early early Pleistocene (no master stream in Egypt)
____________________________________________________
Data from Said (1981)
All available evidence indicate that the Mediteranean sea level was lowered during the late Miocene by several hundred meters. This lowering of the sea level has supporting geomorphological evidence from adjacent land areas where channels cut by streams rejuvenated during the regression of the sea are recorded from all areas drainning into the emptied sea. The Eonile must have eroded its bed deep into the elevated Egyptian plateau along a path which defined the cause of the present day Nile. It must have also received waters from the numerous wadies which debouch into the valley today (Said 1981) (Table 2).
____________________________________________________
Table 2 VOLUME OF SEDIMENTS DEPOSITED BY THE NILE SYSTEM
IN EGYPT
____________________________________________________
Late Miocene EONILE 70,000Km3 in <3.000.000 years
Early Pliocene PALEONILE 70,000Km3 (Possibly from Egypt)
Early Pleistocene PROTONILE ?
Middle Pleistocene PRENILE 100,000Km3
(Atbara connected to the main Nile, >200,000 years ago)
Late Pleistocene NEONILE 1000 connection of the Blue Nile
_____________________________________________________
Modified from Said (1981) .
Said(1981) mentioned that the mineralogy of the pre Nile sediments is radically different from that of the sediments of the earlier river in as much as they include an influx of new minerals such as those characterizing the modern Nile (Shukri 1954). Even though the heavy mineral suite of the Prenile sediments is different from that of the modern Nile, and particularly in the abundance of epidotes relative to pyroxenes, it includes, nevertheless , enough common mineral species to warrant the conclusion that both had a common provenance. The nearest model of drainage that could explain this change of mineral composition is a large influx of water from the Atbara relative to the Blue Nile. Said (1981), showed that the river has undergone great changes since its downcutting in late Miocene time. This is to be expected in view of the complex and varied factors that have influenced its development. He recognized five rivers. These are from the oldest to the youngest: Eonile (late Miocene), Paleonile (late pliocene) , Proto-, Pre-, and Neoniles (Pleistocene). They were separated from one another by episodes in which the river declined, ceased to flow, or radically changed its regimen probably in response to tectonic activity and/or climatic changes.
Adamson (1982) showed that the Nile drainage system originated with the rifting of Africa. He emphasized the fact that tectonic events of great age, probably dating back to the Precambrian, have exerted a powerful influence over the development of the Nile basin and its river system.
Adamson and Williams (1982) are independently in good agreement with Mohr (1974) in the identification of major lineaments in the African Rift Valleys and associated regions of eastern and north eastern Africa. They showed that two sets of sub parallel lineaments are prominent, SE-SSE and ENE. Those two major orientations can be matched to numerous features in East Africa. They relate to the directions of the Red Sea and Gulf of Aden. The Jebel-Marra-Aswa-Mt.Elegon line,the Nile Congo and Cong Chad watersheds.
Issawi (1983) reached the following conclusion ; at the close of the Miocene, there were two main rivers flowing east in Egypt, the Qena River draining most of the northern Eastern Desert and the Allagi river filtering the waters of most of the remainder of this region. No doubt some of these waters found their way into the old trench which is now occupied by the Nile. Waters then draining the highlands of Ethiopia and equatorial Africa were probably ponded within the Sudd region of Sudan south of Khartoum. When the Ethiopian waters succeeded in crossing the catracts in the Nile course ,they reached the old Miocene trench. The new active river drained the huge Sudd region and captured the old rivers, rushing northward towards the Mediterranean . These events took place less than 50,000 years ago, and a new river system developed in North Africa. Although how the rivers were captured , is not well understood , yet the fact that those events are not recorded in Egypt except about 50,000 years ago, is very important.
THE BLUE NILE IN ETHIOPIA
The great Abbai emerging from lake Tana flowing southeast before it turns west and northwest towards Sudan to form the Blue Nile. Williams and Williams (1982) summarised the development of the Blue Nile ; Ethiopia became updomed during Eocene-Oligocene and Miocene-Pliocene times, with major volcanic eruption between about 25 m.y. and 19 m.y ago. Initial entrenchment of the Abbai-Blue Nile and Tekazze-Atbara river systems probably began during the Miocene or very Late Oligocene, and accelerated during Late Pleistocene times, with the eventual removal of some 100 000 km3 of basalt, sandstone, limestone and granite from their basins. Mcdougall et al.,1975) have assumed that this volume of sediments has been deposited in the Nile delta. Gasse et al. (1980) have shown that during the last two to three million years extensive lakes covered parts of the Rift Valley and Afar. Studies at Hadar have shown the continuity of these lakes and of their general climatic setting through nearly a million years.
Adamson and Williams (1980) , in their synthesis of the development of the Ethiopian drainage system ,concluded that the Tertiary uplift along the axes of the Red Sea and Ethiopian Rift arches, together with the rifting itself, disrupted the eastward flowing drainage. The western rim of the rifted arches formed a barrier which reversed the direction of river flow. Drainage over an area of some 180 000 km2 was thus diverted westward into the Nile catchment. The increased elevation caused higher precipitation, greater stream flow , greater stream incision and greater sediment load.
THE WHITE NILE IN EQUATORIA
Doornkamp and Temple (1966),suggested in their investigation of the evolution of part of south Uganda, that the original course of rivers Katonga and Kagera were to the west. Regional earth movements during the Early to Middle Tertiary resulted in swell uplift in eastern and southeastern Uganda and the development of a tectonic depression along the site of the present Albert Rift . Sediment accumulation and continued uplift during the Late Tertiary and Early Pleistocene reversed the westward flow into an easterly direction . Temple (in Williams & Williams ,1980) considered that the modern lake Victoria basin is no older than late Middle Pleistocene on the grounds that it post-dates Miocene and Pliocene earth movements in the Kavirondo Gulf and in the Albert Rift, and contains sediments no older than the late Middle Pleistocene Nsongezi Series which have late Acheulian implements near the top of the sequence. Williams and Williams (1980) cautioned against regarding Lake Victoria as Pleistocene , Lake Albert as Mio-Pliocene and Ugandan drainage through into southern Sudan as Late Cenozoic. The presence of lake deposits up to 100 m above the present level of lake Victoria discovered by Doornkamp & Temple (1966), definitely prove the damming of the Ugandan drainage , at least to the 12,500 date given by Livingstone (1980), and supported by Adamson and Williams (1980), who postulated that it is possible that Lake Victoria itself had never supplied water to the Nile prior to 12 500 B.P.
Adamson and Williams (1980), agreed with Cooke (1958) that the Kafu, Katonga and Kagera rivers flowed westwards across Uganda before the Rift influence , and that the present positions of Lakes Victoria and Albert were within the Zaire drainage system and ultimately discharging into the Atlantic Ocean (Fig 10.3).
Adamson (1982) showed that the Nile drainage system originated with the rifting of Africa. He emphasized the fact that tectonic events of great age, probably dating back to the Precambrian, have exerted a powerful influence over the development of the Nile basin and its river system.
THE INTEGRATED NILE
There are two theories in relation to the age of an integrated Nile. The first one is that the integrated drainage of the Nile basin is of young age. De Heinzelin (1968) proposed that the Nile basin was formerly broken into series of separate basins, only the most northerly (the Proto Nile basin) feeding a river following the present course of the Nile in Egypt and in the far north of the Sudan. He postulated that the more southerly basins drained either into the Red Sea, the Atlantic Ocean or internally. Butzer and Hansen (1968) concluded that the White Nile and Blue Nile basins certainly did not merge with the Saharan Nile before the early Pleistocene. Wendorf & Schild (1976) agree with Butzer and Hansen (1968) conclusions, and differ with De Heinzelin in his assumption of a late connection of the Ethiopian basin. Said (1981) supports De Heinzelin and gives further evidence that the Nile in Egypt is of local sources. Said (1981) stresses the fact that Egypt itself supplied most of the waters of the Nile during the early part of its history. He also concluded that arid conditions did not set in over the Sahara in general and Egypt in particular except during the Pleistocene. Previous to this epoch and during most of the Cenozoic, there is evidence that the climate in Egypt was wet. There was a good mat of vegetation, little surface denudation, and during several epochs, moderate to intense chemical weathering.
The other theory is that the drainage from Ethiopia via rivers equivalent to th Blue Nile and the Atbara/Takazze flowed to the Mediterranean via the Egyptian Nile since well back into Tertiary times (McDougall et al. 1975, Williams & Williams 1980), Adamson (1982) concluded that the Nile has been supplying Ethiopian water and sediment to the Mediterranean since well back into the Tertiary, that is from several to many millions of years. He accepted the idea that several basins might have been present at the early stage, but the coincidence between the volume of debris eroded from Ethiopia and the volume of the Nile cone in the Mediterranean, argue for very early integration of sub Saharan drainage with the Egyptian Nile.
LATE QUATERNARY RIVER NILE
Adamson (1982), concluded that throughout the late Quaternary, the Nile behaved as an integrated river system with events downstream being determined by those in the headwaters. The characteristics of water discharge and sediment load which originated in the headwaters, in turn controlled the nature of aggradation, erosion and flooding on the low angle flood-plains far downstream. He clearly showed two important periods; the late Pleistocene arid period c.20,000 to c.13,000 BP and the terminal Pleistocene-mid-Holocene moist period c.12,000 to c.5,000 BP .
Butzer et al. (1972) in their study of the fluctuations of Lakes Rudolf, Nakuru, Naivasha, Magadi, Rukwa, Chad and Victoria , showed that all those lakes were high in the early Holocene between 10,000 and 8,000 years ago . They showed that transgressions leading to this high stand began about 12,000 years ago, and evidence from three basins (Victoria, Nakuru, and Chad) indicates a pause or minor recession just at or before 10,000 years ago. Wherever information is available for the period preceding 12,000 years ago , it has consistently been shown that lakes are much smaller.
Gasse et al. 1978, have shown that three lacustral phases can be recognized in Abhe; first the 60,000 years ago, second the 40,000 to 30,000 years ago and the third 29,000 to 17,000 years ago. They also showed that all the lakes transgressed rapidly after 11,000-10,000 y B.P. reaching maximum elevations towards 9400 y B.P. Gasse et al. (1980) mentioned that the discovery of large tropical lakes at Hadar from 3.3 to 2.6 m.y has confirmed the picture emerging from East Africa (Olduvai, Omo, East Turkana, etc). The site has yielded an exceptionally fine collection of fossil remains, all of which indicate, as in East Africa, a Sudanian or Sahelian climate. Studies at Hadar have shown the continuity of these lakes and of their general climatic setting through nearly a million years. This period contrasts strikingly with the following one. Aridity set in between 2.5 and 2 m.y in the Omo(based on the pollen and rodents) and in the Afar, where it probably marks the onset of desert conditions. Williams and Adamson (1973), made the following summation; About 12,000 years BP Lake Victoria overflowed, the level of the White Nile rose, and the dunes between Kosti and Ed Dueim became partly buried by alluvial clay. After 8,000 yr BP the level of the White Nile fell, dark clays accumulated in the swamps bordering the river until about 4,000 yr BP. The fixed dunes of Kordofan and the even flow of the modern White Nile indicate the late Pleistocene dry period was more arid than the present semi arid climate of central Sudan , and it is possible that the White Nile may have ceased to flow into the main Nile, which would thus have been more seasonal in its regime than now.
THE RIVER NILE IN SUDAN
Williams (1963), Williams and Adamson (1968), Adamson and Williams (1972), Williams and Williams (1980), Adamson and Williams (1980) and Adamson (1982) were the pioneers who studied the evolution of the River Nile in Sudan. Their work covered most of the central Sudan, including the southern part in the Southern provinces ; the Southern Sudanese trough (Adamson and Williams,1980).
THE EVOLUTION OF THE RIVER NILE AND THE BURIED SALINE RIFT LAKES IN SUDAN
From the discussion given in the previous pages, and from the discussions and evidence presented in this page, the writer is lead to accept the first theory which postulates that the integrated River Nile is of young age . The following conclusions support this case :
a) Each one of the rift systems , was a closed basin
b) That these basins were not interconnected , except after their subsidence ceased , and the rate of sediment deposition was enough to fill up the basins to such a level that would allow connection to take place.
c) That the Nuba basin was connected to both the Gezira basin and the Sudd basin, this can be shown from the cross sections . The contour of 380 m above mean sea level, passing parallel to the White Nile , on the east and west banks, from south of Khartoum to Jebelin, makes all this area of the White Nile lower than the Blue Nile at any point south from Hasaheisa. It would be natural , with a gradient much higher than the existing White Nile gradients, that the Blue Nile would move westward towards the White Nile , especially if the exit from Sabaloka was choked or at a higher level. Which means that the paleo channels of the Blue Nile which shows a southwesterly bend, were all moving towards the Abu Habil trough in the White Nile rift. Another proof of the movement of the Nile southward is the discovery of pure glassy volcanic ash by Abdalla and Adamson reported in Adamson et al (1982) , 1.6 m below the surface of Holocene White Nile clays south of Kawa on the White Nile. This volcanic ash is similar to the ash deposits discovered along the Blue Nile north of Wad Medani. These workers reached the conclusion that this ash is almost certainly entered the White Nile from the Sobat river or from the seasonal streams which drain the Ethiopian highs north of the Sobat. They also concluded that the deposits are possibly younger than the Blue Nile deposits , since they lie below a skin of Holocene clays. But from the new evidence available, it is now possible to interpret the presence of the volcanic ash in the White Nile as being deposited by the Blue Nile. There is nothing to prevent the Blue Nile waters from reaching this spot, as shown by the gradients of the Blue and White Niles. Also the interpretation of the age as younger than the Blue Nile ashes, based upon the thin thickness of the Holocene deposits, can be interpreted by the fact that the sedimentation rate of the White Nile rift is much slower; that is the reason for the shallow cover. Although Adamson and Williams (1982) gave an age of 25,000 years by dating carbonate deposits above them , it is most likely that the volcanic ashes were brought about by the wet period of 70,000 years ago.
d) This connection between the Gezira and Nuba basins , means that the overflow from the Blue Nile would move southward towards the Nuba basin and not northward owards the Sabaloka.
e) From the inscriptions on the rocks of Semna catract (about 70 kms south of Wadi Halfa) (Wilcocks,1904 ; Ball,1939) , it was assumed that the River Nile lowered its channel in the rocky barrier by about 8 m within the last 3800 years. That means about 0.0021 m/year . It is postulated here that the Semna catract was rising at this rate, and that the Nile was cutting its way through its trapped course. It is also postulated that all the catracts along the course of the River Nile are uplifts, adjacent to subsiding basins. It is not a coincidence that all the catracts are the only Basement outcrops along the course of the River Nile. They are the result of isostatic adjustment of the subsiding basins . From the natural gradient northward, the flow started to move northward following the Tertiary rifting patterns; NW-SE and SW-NE. Due to the compensational effects, the uplifted areas started to rise, this rise in the area between the Sabaloka cataract in the south to the Semna catract in the north started only 70,000-80,000 years ago. This can be calculated using the uplift rate from the Semna catract. This age agrees with the date given by Said (1981) for the connection of the Blue Nile to the main River Nile in Egypt. It also agrees with the wet periods of Gasse (1977), which supports the case for an increase in sedimentation during the wet periods, filling of the basins and overflowing northward , so as to connect with the northern river system.
f) The Darfur dome in the north was the controlling ridge, that distributed the northerly and southerly water shed areas. As mentioned earlier, the Darfur dome highest point was in the west in J. Marra area and that this elevation decreases in an easterly direction . It was also shown that the intensity of rifting decreased in an eastward direction; which means that the River Atbara rift has the shallowest grabens.
g) Due to this difference in elevation, and to the difference in graben size to be filled by sediments , it is logical to expect that the rivers in the eastern part will be the first rivers to connect to the Egyptian drainage system. The River Atbara, was the first river to fillup its subsiding graben and moved northward towards the main River Nile.
h) The Blue Nile was always overflowing towards the Nuba basin . It was not connected to the main Nile before 70,000-80,000 years ago. This connection occured due to the damming effect of the outlet towards the south by sand dunes during arid periods .
i) The White Nile system , in Bahr El Arab and White Nile rift areas , remained a closed lake until , the connection of the Victoria Nile some 12500 years ago. This can easily be proven using the hydrological budget, if the losses from the White Nile basin now are about 42 km3 annually, and if the Victoria Nile is cutoff, then there would be no outflow from the Sudd. The other proof is the presence of the evaporites on the Nuba lakes area, and all the areas, extending from south of Ed Dueim to near Renk which indicates that the troughs and grabens of the White Nile rift were occupied by a saline standing water body.
j) At the same time the present existence of the Sudd and the Machar marches in Bahr El Arab and the White Nile rifts systems, indicate that these areas are lower than the normal gradient of the area to the north . These observations may indicate that the two areas are not yet fully filled with sediments. Another possibility is that these areas are still active, but are subsiding at a slow rate. This might agree with the continuous tremors occuring at J. El Rajaf near Juba , at the southern edge of Bahr El Arab rift (Whiteman 1971 , personal observation ,1975) for Wadi El Melik and Wadi El Muggadam. The second one in Wadi El Qaab , west of Dongola, where the evaporites extends on the surface of depression for more than ten kilometers. This used to be the lake area of Wadi Howar. This shows , that this phenomena of closed lakes system is existing northward and thus complete the pattern of the Sudanese lakes northward to the borders of Egypt.
l) The filling up of the saline lakes, or depressions, led to the connection of the Egyptian Nile with the Sudanese Nile, which captured the Ethiopian and Equatorial head waters during the tectonic activities that formed the rift structures of Eastern and Central Africa and The Sudanese Rift Systems.
m) From all the above discussions it can be postulated , after making all the required reservations , that the age of the integrated River Nile is very recent. The River Atbara overflowed its closed basin about 100,000 to 120,000 years B.P , The Blue Nile overflowed its closed basin 70,000 to 80,000 years B.P and the White Nile overflowed its closed basin 12,500 years B.P.
